Piezoelectric single crystal device and fabrication method thereof

ABSTRACT

The present invention provides a piezoelectric single crystal device excellent in heat resistance and capable of stably maintaining the electromechanical coupling factor k 31  in a lateral vibration mode at a high value of 50% or more without a decrease even in an operating environment in which the temperature changes from room temperature to a high temperature (specifically, 150° C.), and also provides a fabrication method thereof. Specifically, assuming that the [010] axis of a tetragonal system having the [001] axis as a C axis (with the largest lattice constant) is a polarization direction 3, a normal direction 1 to an edge face T of the piezoelectric device is within the solid-angle range of ±25° with respect to the [-101] axis substantially orthogonal to the polarization direction 3, the range including the [-101] axis. Assuming that the [011] axis of the tetragonal system is the polarization direction 3, the normal direction 1 to the edge face T of the piezoelectric device is within the solid-angle range of ±25° with respect to the [0-11] axis substantially orthogonal to the polarization direction 3, the range including the [0-11] axis. In any case, the electromechanical coupling factor k 31  in the direction orthogonal to the polarization direction 3, i.e., in the lateral vibration mode, is 50% or more.

TECHNICAL FIELD

The present invention relates to a piezoelectric single crystal deviceand a fabrication method thereof. More specifically, the inventionrelates to a piezoelectric single crystal device excellent in heatresistance and capable of stably maintaining the electromechanicalcoupling factor k₃₁ in a lateral vibration mode in a directionsubstantially orthogonal to a polarization direction at a high value of50% or more without a decrease even in a high-temperature (specifically,150° C.) operating environment, the device being composed of apiezoelectric single crystal device material with a tetragonal-systemcomplex perovskite structure, which is a solid solution (referred to as“PMN-PT” or PMNT”) represented by Pb[(Mg, Nb)_(1-X)Ti_(X)]O₃ containinglead magnesium niobate Pb(Mg, Nb)O₃ and lead titanate PbTiO₃. Theinvention also relates to a fabrication method of the piezoelectricsingle crystal device.

BACKGROUND ART

For example, as shown in FIG. 1, with a rectangular plate (a/b≧2.5,a>>L, b>>L) having an aspect ratio (a/b) of 2.5 or more, theelectromechanical coupling factor k₃₁ in the lateral vibration mode isproportional to the square root of the conversion efficiency of electricenergy or mechanical energy for the magnitude of vibration (lateralvibration) in a direction 1 orthogonal to a polarization direction 3 atthe time of applying a voltage in the polarization direction 3. Thehigher this value, the more the efficiency improves. It should be notedthat the shape of a piezoelectric single crystal device may be a squareshape, a disc shape, a rod shape, or the like, instead of theabove-described rectangular plate shape, and the electromechanicalcoupling factor k₃₁ can be determined in the same way with any one ofthese shapes.

In general, lead zircon titanate Pb(Zr, Ti)O₃ (PZT) disclosed in T.Ogawa, M. Matsushita, Y. Tachi, and K. Echizenya, “Program Summary andExtended Abstracts of the 10th US-Japan Seminar on Dielectric andPiezoelectric Ceramics” (Sep. 26-29, (2001), pp. 245-248) is widely usedas a material for forming the piezoelectric device. However, lead zircontitanate (PZT) described in the document of Ogawa et al has anelectromechanical coupling factor k₃₁ of about 30%.

In order to obtain higher k₃₁ than that of the above-described PZT, forexample, Japanese Unexamined Patent Application Publication No.11-171644 discloses a piezoelectric ceramic composition containingx(Pb₂Me₂O₇)½·(1-x)[Pb(Zr_(1-y)Ti_(y))O₃] as a main component and Cr andSi secondary components. However, the piezoelectric ceramic compositiondisclosed in Japanese Unexamined Patent Application Publication No.11-171644 has an electromechanical coupling factor k₃₁ of 40% or less.

Furthermore, Jpn. J. Appl. Phys. 90(2001) (pp. 3471-3475) discloses themeasured piezoelectric properties of a single crystal of0.67Pb(Mg_(1/3)Nb_(2/3))O₃-0.33PbTiO₃ having an electromechanicalcoupling factor k₃₁ of as high as 59% (0.59), such as lateral mode k₃₁in the [100] or [010] direction orthogonal to the [001] direction as thepolarization direction.

However, Jpn. J. Appl. Phys. 90(2001) (pp. 3471-3475) discloses theelectromechanical coupling factor k₃₁ measured at room temperature, butthe electromechanical coupling factor k₃₁ in use in a high-temperature(specifically 150° C.) environment is not disclosed. For example, in useas a piezoelectric device, the piezoelectric device may be soldered orbonded to a resin and thus used in an environment in which thetemperature changes from room temperature to a high temperature(specifically 150° C.). In this case, the piezoelectric device tends tobe decreased in electromechanical coupling factor k₃₁ due todeterioration. This point is not disclosed or suggested in Jpn. J. Appl.Phys. 90(2001) (pp. 3471-3475).

DISCLOSURE OF INVENTION

An object of the present invention is to provide a piezoelectric singlecrystal device excellent in heat resistance and capable of stablymaintaining the electromechanical coupling factor k₃₁ at a high value of50% or more without a decrease even in an operating environment in whichthe temperature changes from room temperature to a high temperature(specifically 150° C.), the device being composed of a piezoelectricsingle crystal device material having a tetragonal-system complexperovskite structure, which is a solid solution (referred to as “PMN-PT”or PMNT”) represented by Pb[(Mg, Nb)_(1-X)Ti_(X)]O₃ containing leadmagnesium niobate and lead titanate, and also provide a fabricationmethod of the piezoelectric single crystal device.

In order to achieve the object, the gist of the present invention is asfollows:

(1) A piezoelectric single crystal device, with a polarization directionalong a [101] axis of a tetragonal system which has a [001] axis as a Caxis (with the largest lattice constant), has a normal direction to anedge face thereof within the solid-angle range of ±25° with respect to a[-101] axis which is substantially orthogonal to the polarizationdirection, the range including the [-101] axis, and yields anelectromechanical coupling factor k₃₁ of 50% or more in a lateralvibration mode in a direction substantially orthogonal to thepolarization direction.

(2) A piezoelectric single crystal device, with a polarization directionalong a [011] axis of a tetragonal system which has a [001] axis as a Caxis (with the largest lattice constant), has a normal direction to anedge face thereof within the solid-angle range of ±25° with respect to a[0-11] axis which is substantially orthogonal to the polarizationdirection, the range including the [0-11] axis, and yields anelectromechanical coupling factor k₃₁ of 50% or more in a lateralvibration mode in a direction substantially orthogonal to thepolarization direction.

(3) The piezoelectric single crystal device described above in (1) or(2) is composed of a single crystal device material which is a solidsolution of Pb[(Mg, Nb)_(1-X)Ti_(X])O₃ (wherein X is a molar fraction ofTi relative to the total molar fraction of 1 of Mg, Nb, and Ti), Xsatisfying the relation 0.30<X<0.40 and the material having a complexperovskite structure.

The tetragonal system has a parallelepiped unit lattice having a crystalstructure in which the [100] axis (a axis) and the [010] axis (b axis)having equal lengths are orthogonal to the [001] axis (c axis) havingthe largest lattice constant. When the molar fraction of lead titanate(PT) in the solid solution described above in (3) is close to 0.30, thestructure includes a portion showing a pseudocubic system which isthermodynamically a low-temperature phase.

The perovskite structure means a structure (RMO₃) of a unit lattice of asolid solution single crystal in which R ions are positioned at thecorners of the unit lattice, oxygen ions are positioned at the facecenters of the unit lattice, and a M ion is positioned at the bodycenter of the unit lattice, as schematically shown in FIG. 2. Thecomplex perovskite structure according to the present invention meansthat any one of two or more types of M ions, not a single element ion,is positioned at the body center shown in FIG. 2.

(4) The piezoelectric single crystal device described above in (1) to(3) further contains 0.05 mol % to 30 mol % of In in the solid solution.

(5) A method for fabricating the piezoelectric single crystal devicedescribed in any one of (1) to (4) includes primary polarizationprocessing for polarizing a single crystal ingot, a cut out singlecrystal block, or a cut out single crystal device by applying anelectric field under predetermined conditions in the polarizationdirection along a [101] or [011] direction before or after processingfor cutting out a single crystal device material in a predeterminedshape from the single crystal ingot in a predetermined direction.

(6) The method for fabricating the piezoelectric single crystal devicedescribed above in (5) includes primary polarization processing forpolarizing the single crystal ingot or the single crystal block byapplying an electric field in the [101] or [011] direction underpredetermined conditions, and processing for cutting out a singlecrystal device of a predetermined shape in a predetermined directionfrom the single crystal ingot or the single crystal block.

(7) In the method for fabricating the piezoelectric single crystaldevice described above in (5) or (6), the primary polarizationprocessing includes applying a DC electric field of 350 to 1500 V/mm inthe temperature range of 20° C. to 200° C. in the [101] or [011]direction of the single crystal ingot or the single crystal block orcooling the single crystal ingot or the single crystal block to roomtemperature while applying a DC electric field of 250 to 500 V/mm at atemperature higher than the Curie temperature (Tc) thereof.

(8) The method for fabricating the piezoelectric single crystal devicedescribed above in (5) includes processing for cutting out a singlecrystal device of a predetermined shape in a predetermined directionfrom the single crystal ingot, and primary polarization processing forpolarizing the single crystal device by applying an electric field inthe [101] or [011] direction under predetermined conditions.

(9) In the method for fabricating the piezoelectric single crystaldevice described above in (5) or (8), the primary polarizationprocessing includes applying a DC electric field of 350 to 1500 V/mm inthe temperature range of 20° C. to 200° C. in the [101] or [011]direction of the cut out single crystal device or cooling the singlecrystal device to room temperature while applying a DC electric field of250 to 500 V/mm at a temperature higher than the Curie temperature (Tc)thereof.

(10) The method for fabricating the piezoelectric single crystal devicedescribed above in (5) to (9) further includes auxiliary polarizationprocessing for polarizing by applying an electric field in a directionorthogonal to the polarization direction before or after the primarypolarization processing.

Examples of the electric field applied in the direction 1 orthogonal tothe polarization direction 3 include steady-state electric fields suchas DC electric fields, pulse electric fields, and AC electric fields;and attenuation electric fields. The conditions such as the strength,application time, temperature, and the like of the electric field areappropriately controlled according to the properties of eachpiezoelectric crystal device and the desired value of theelectromechanical coupling factor k₃₁ in the direction 1 orthogonal tothe polarization direction 3. These conditions can be determined by anexperiment or the like. As the pulse electric field, unipolar andbipolar pulses such as a square wave and an AC triangular wave can beused.

The present invention is capable of fabricating the piezoelectric singlecrystal device used for applications positively using theelectromechanical coupling factor k₃₁ in the direction 1 (lateralvibration mode) orthogonal to the polarization direction 3, for example,an accurate positioning actuator for a magnetic head, a piezoelectricgyro device, an image stabilizer for a digital still camera, a cardiacpacemaker sensor, and the like. In particular, the piezoelectric singlecrystal device of the present invention can stably maintain theelectromechanical coupling factor k₃₁ in the lateral vibration mode at ahigh value of 50% or more without a reduction even in an operatingenvironment in which the temperature changes from room temperature to ahigh temperature (specifically 150° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the orientation and shape of apiezoelectric single crystal device in a polarization state according tothe present invention.

FIG. 2 is a schematic perspective view of a perovskite crystal structure(RMO₃).

FIG. 3 is a drawing showing various shapes of the edge faces of apiezoelectric single crystal device using a lateral vibration modeaccording to the present invention.

FIG. 4 is a phase diagram of PMN-PT (PMNT).

FIG. 5 is a perspective view showing, in a three-axis orthogonalcoordinate system, a piezoelectric single crystal device 10A cut outwith the (101) face as a wafer face (the widest face).

FIG. 6 is a perspective view showing, in a three-axis orthogonalcoordinate system, a piezoelectric single crystal device 10B cut outwith the (011) face as a wafer face (the widest face).

FIG. 7 is a waveform diagram of a bipolar triangular wave pulse.

FIG. 8A is a diagram illustrating the application of a DC electric fieldto a piezoelectric single crystal device 10A.

FIG. 8B is a diagram illustrating the application of a DC electric fieldto a piezoelectric single crystal device 10B.

FIG. 9A is a diagram illustrating the direction of cutting out apiezoelectric single crystal device 10A from a single crystal wafer 11in which the normal direction 1 to the edge face 10 c (or T) of thepiezoelectric device is in the range of 0 to 90°.

FIG. 9B is a diagram illustrating the direction of cutting out apiezoelectric single crystal device 10B from a single crystal wafer 11in which the normal direction 1 to the edge face 10 c (or T) of thepiezoelectric device is in the range of 0 to 90°.

FIG. 10 is a plot diagram showing changes in the dielectric constant ofeach of piezoelectric single crystal devices 10A and 10B of the presentinvention with temperature, in which the molar fraction X of Tisatisfies 0.30<X<0.40.

FIG. 11 is a plot diagram showing changes in the dielectric constant ofeach of piezoelectric single crystal devices 10A and 10B of the presentinvention with temperature, in which the molar fraction X of Ti is 0.30or less.

FIG. 12A is a plot diagram showing changes in the electromechanicalcoupling factor k₃₁ of a piezoelectric single crystal device 10A of thepresent invention in repeated heat cycle tests, in which the molarfraction X of Ti satisfies 0.30<X<0.40.

FIG. 12B is a plot diagram showing changes in the electromechanicalcoupling factor k₃₁ of a piezoelectric single crystal device 10B of thepresent invention in repeated heat cycle tests, in which the molarfraction X of Ti satisfies 0.30<X<0.40.

FIG. 13 is a plot diagram showing changes in the electromechanicalcoupling factor k₃₁ of each of piezoelectric single crystal devices 10Aand 10B in repeated heat cycle tests, in which the molar fraction X ofTi is 0.30 or less.

REFERENCE NUMERALS

10 piezoelectric single crystal device

10A top face (or electrode face) of piezoelectric single crystal device

10 b bottom face (or electrode face) of piezoelectric single crystaldevice

10 c or T edge face of piezoelectric single crystal device using alateral vibration mode

11 single crystal wafer

10A piezoelectric single crystal device with [101] direction aspolarization direction

10B piezoelectric single crystal device with [011] direction aspolarization direction

a dimension in lateral direction (lateral vibration direction 1) ofpiezoelectric single crystal device

b dimension of edge face (depth direction 2) of piezoelectric singlecrystal device

b′ convex edge face of piezoelectric single crystal device

b″ concave edge face of piezoelectric single crystal device

L dimension in longitudinal direction (polarization direction 3) ofpiezoelectric single crystal device

V DC voltage

1 normal direction (lateral vibration direction) to edge face ofpiezoelectric single crystal device

3 polarization direction (longitudinal vibration direction)

BEST MODE FOR CARRYING OUT THE INVENTION

The reasons for restricting a piezoelectric single crystal device of thepresent invention will be described below.

(1) Relation Between Polarization Direction and Normal Direction toPiezoelectric Device Edge Face:

As shown in FIG. 5, assuming that the [101] axis of a tetragonal systemhaving the [001] axis as a C axis (with the largest lattice constant) isa polarization direction, the normal direction 1 to the edge face T of apiezoelectric single crystal device 10A is preferably within a conicalsolid-angle range of ±25° with respect to the [-101] axis substantiallyorthogonal to the polarization direction [101], the range including the[-101] axis. Alternatively, as shown in FIG. 6, assuming that the [011]axis of a tetragonal system having the [001] axis is the polarizationdirection, the normal direction 1 to the edge face T of thepiezoelectric single crystal device 10A is preferably within a conicalsolid-angle range of ±25° with respect to the [0-11] axis substantiallyorthogonal to the polarization direction [011], the range including the[0-11] axis. As shown in FIGS. 5 and 6, the normal direction n to thewidest face of the piezoelectric single crystal device is within theconical solid-angle range of 0° ±25° on the assumption that thepolarization direction [101] or [011] is 0°.

The conceivable reason for restricting the normal direction 1 to theedge face T of the piezoelectric single crystal device using a lateralvibration mode within this angle range is as follows: Within thespecified solid-angle range, lateral vibration in the [-101] axialdirection or lateral vibration in the [0-11] axial direction does notdisperse in other axial directions, and thus energy in the lateralvibration mode in the axial direction is maintained without beingdecreased, resulting in a high electromechanical coupling factor k₃₁ of50% or more. On the other hand, when the normal direction 1 to the edgeface T of the piezoelectric single crystal device is out of thespecified solid-angle range, in the former piezoelectric single crystaldevice 10A, lateral vibration is dispersed by the influence of the[-111] or [-1-11] axis at an angle of about 350 with the [-101] axialdirection. In the latter piezoelectric single crystal device 10B,lateral vibration is dispersed by the influence of the [1-11] or [-1-11]axis at an angle of about 35° with the [0-11] axial direction. Thismeans that energy in the lateral vibration mode in the [-101] or [0-11]direction is decreased. As a result, an electromechanical couplingfactor k₃₁ of 50% or more in the lateral vibration mode cannot beobtained. The piezoelectric single crystal device 10A shown in FIG. 5and the piezoelectric single crystal device 10B shown in FIG. 6 areequivalent in view of the symmetry of the tetragonal system.

(2) Crystal Structure of Piezoelectric Single Crystal Device (TetragonalComplex Perovskite Structure):

The crystal structure of the present invention is a tetragonalstructure. The tetragonal structure has a parallelepiped unit latticehaving a crystal structure in which the [100] axis (a axis) and the[010] axis (b axis) with equal lengths are orthogonal to the [001] axis(c axis) with the largest lattice constant. However, when the molarfraction of lead titanate (PT) in the solid solution of Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ described above in (3) is close to 0.30, thestructure includes a portion of a pseudocubic system which isthermodynamically a low-temperature phase.

The crystal structure of the present invention is a complex perovskitestructure (RMO₃) in which in the unit lattice shown in FIG. 2, Pb ionsare positioned at the corners of the unit lattice, oxygen ions arepositioned at the face centers of the unit lattice, and a M ion of Mg,Nb, In, or Ti is positioned at the body center of the unit lattice.

(3) Composition of Single Crystal Device

The composition of the single crystal device of the present invention iscomposed of the solid solution of, for example, Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ (wherein X is the molar fraction of Ti relative tothe total molar fraction of 1 of Mg, Nb, and Ti) in which X satisfies0.30<X<0.40, and has the complex perovskite structure. It is morepreferably that 0.34<X<0.38. When the molar fraction X is 0.3 or less,as shown in FIG. 4, the solid-solution device is mainly occupied by apseudocubic system. In addition, when the device is used in atemperature range from room temperature to a high temperature (e.g.,150° C.), phase transition easily occurs between the tetragonal systemand the pseudocubic system because the phase transition temperatureT_(rt) is a relatively low temperature higher than room temperature. Asa result, the performance of the tetragonal piezoelectric device may bedegraded to decrease the electromechanical coupling factor k₃₁. When themolar fraction X is 0.40 or more, the phase transition temperaturebetween the pseudocubic system and the tetragonal system is lower thanroom temperature, and thus phase transition does not occur in the rangeof room temperature to 150° C. However, the molar fraction of leadtitanate is excessively high, and thus the piezoelectric properties ofthe solid solution tend to deteriorate, thereby failing to obtain anelectromechanical coupling factor k₃₁ or 50% or more in the lateralvibration mode.

When the molar ratio Mg/Nb of Mg to Nb in lead magnesium niobate Pb(Mg,Nb)O₃ is in a range of 0.45 to 0.54, the complex perovskite structure ismaintained. Therefore, this range is included in the present invention.

For the piezoelectric device of the present invention, lead indiummagnesium niobate-lead titanate (PIMN-PT) containing lead magnesiumniobate-lead titanate (PMN-PT) and further containing In, preferably0.05 mol % to 30 mol % of In, can be used. Since the ionic radius ofindium (In) is larger than that of magnesium (Mg) but is smaller thanthat of niobium (Nb), lattice strain due to a difference in ionic radiumbetween niobium (Nb) and magnesium (Mg), each of which is positioned atthe body center of the unit lattice of the perovskite structure, isreduced, thereby causing the function to prevent the occurrence ofcracks in single crystal growth and chipping in piezoelectric deviceprocessing. In the present invention, therefore, the adding amount ofindium necessary for exhibiting the function is 0.05 mol % or more, butthe addition of over 30 mol % is undesirable because the melting pointof a raw material in single crystal growth is increased to causedifficulty in processing control in fabrication.

In the case of the need to increase the specific inductive capacityε_(r), 0.5 mol ppm to 5 mol % of at least one element of Sb, La, W, andTa may be further added to the composition of the piezoelectric singlecrystal device. In the case of the need to increase the mechanicalquality factor Qm, 0.5 ppm to 5 mol % of at least one of Mn and Cr maybe further added to the composition of the piezoelectric single crystaldevice.

Furthermore, Al and Li contribute to stabilization in single crystalgrowth. In order to obtain the effect, at least one of Al and Li ispreferably added in a total of 0.05 mol % or more.

These atoms (Sb, La, W, Ta, Mn, Cr, Al, Li) are positioned at the bodycenters of the unit lattices or positioned between the lattices. Theaddition of the atoms in a total of over 5 mol % may cause difficulty inobtaining a single crystal, and thus a polycrystal may be produced.

When calcium oxide is added to a raw material, calcium (Ca) of calciumoxide is positioned as substitutional atoms at some of the lead (Pb)sites (R ions in FIG. 2) in a crystal lattice composed of a solidsolution of lead-based perovskite structure compound (lead magnesiumniobate, lead titanate, and lead indium niobate) and thus function tosuppress the evaporation of lead oxide at a high temperature. Thefunction of Ca suppresses the production of a pyrochlore phase, therebyfacilitating the production of a single crystal of a desired complexperovskite phase. In the present invention, in order to exhibit the Cafunction, it is necessary to substitute by calcium at a ratio of 0.05mol % or more. However, substitution at a ratio of over 10 mol % causesdifficulty in single crystal growth. Therefore, 0.05 mol % to 10 mol %of lead in the crystal lattice is preferably substituted by calcium, and0.05 mol % to 5 mol % of lead is more preferably substituted by calcium.

In order to substitute 0.05 to 10 mol % of lead in the composition(crystal lattice) of a single crystal ingot by calcium, it is necessaryto add calcium in view of the evaporation of calcium in single crystalgrowth. A method for adding calcium is not particularly limited. Forexample, calcium-substituted lead magnesium niobate, calcium-substitutedlead zinc niobate, or calcium-substituted lead titanate may be used.Alternatively calcium oxide or calcium carbonate may be added to the rawmaterial.

Impurities such as Fe, Pt, Au, Pd, and Rh may be mixed in from the rawmaterial and crucible used in the process for producing thepiezoelectric single crystal. However, these impurities inhibit theproduction of the single crystal and are preferably suppressed to atotal of 0.5 mol % or less.

(4) Formation of Piezoelectric Single Crystal Device

The shape of the intended piezoelectric single crystal device of thepresent invention is preferably such a rectangular plate as shown inFIG. 1 from the viewpoint that the electromechanical coupling factor k₃₁in the direction 1 (lateral vibration mode) substantially orthogonal tothe polarization direction 3 is effectively increased. In particular,the shape of the device is preferably a rectangular plate having anaspect ratio (a/b) of 2.5 or more (a/b≧2.5, a>>L, b>>L) and morepreferably a rectangular plate having an aspect ratio (a/b) of 3 ormore. The both edge faces (short sides b) of the rectangular plate ofthe present invention may be convexly curved b′ (broken line) orconcavely curved b″ (one-dot chain line) according to applications, asshown in FIG. 3. The shape may be a square plate with a=b. In thepresent invention, the edge faces of the piezoelectric device are shownby the short sides b orthogonal to long sides a in a plan view of FIG.3. Therefore, the normal direction 1 to the edge faces of thepiezoelectric device is parallel to the long sides a of thepiezoelectric device.

Next, a preferred method for fabricating the piezoelectric singlecrystal device of the present invention will be described.

A method for fabricating the piezoelectric single crystal device of thepresent invention includes primary polarization processing forpolarizing a single crystal ingot or a single crystal block having atetragonal structure by applying an electric field in a [101] or [011]direction thereof under predetermined conditions, and processing forcutting out the single crystal device in a predetermined shape from thesingle crystal ingot in a predetermined direction.

Another method for fabricating the piezoelectric single crystal deviceof the present invention includes processing for cutting out the singlecrystal device in a predetermined shape from a single crystal ingot witha tetragonal structure in a predetermined direction, and primarypolarization processing for polarizing the single crystal device byapplying an electric field in the [101] or [011] direction thereof underpredetermined conditions.

The single crystal block is cut into a block shape from the singlecrystal ingot by a wire saw or the like. When the polarizationprocessing is difficult in view of the shape of the single crystalingot, the single crystal block easy to polarize is cut out andsubjected to the polarization processing.

Now, description will be made of the reasons for restricting thefabricating method of the invention in each of the processings.

(1) Fabrication of Single Crystal Ingot:

A single crystal which is a solid solution composed of Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ (wherein X is the molar fraction of Ti relative tothe total molar fraction of 1 of Mg, Nb, and Ti) in which X satisfies0.30<X<0.40, a single crystal having the above-described composition andfurther containing 0.05 to 30 mol % of In and 0.5 mol ppm to 5 mol % ofat least one of Mn, Cr, Sb, W, Al, La, Li, and Ta, or a single crystalhaving the above-described composition in which 0.05 to 10 mol % of leadis substituted by calcium is used. An ingot of the single crystal isproduced by a method in which a raw material prepared to have the abovecomposition is dissolved in a flux and then coagulated by decreasing thetemperature, or a method in which the raw material is melted by heatingto above the melting point and then coagulated in one direction.Examples of the former method include a solution Bridgemann method andTSSG method (Top Seeded Solution Growth). Examples of the latter methodinclude a melt Bridgemann method and a CZ method (Czochralski method).In the present invention, the production method is not particularlylimited.

(2) Determination of Crystallographic Orientation of Single CrystalIngot:

The [101] axial orientation or [011] axial orientation of the singlecrystal ingot is approximately determined by a Laue method. At the sametime, the [-101] axial orientation substantially orthogonal to the [101]axial orientation and the [010] axial orientation, or the [0-11] axialorientation substantially orthogonal to the [011] axial orientation andthe [100] axial orientation are approximately determined. As shown inFIGS. 5 and 6, the [101] axis and the [011] axis are equivalent from theviewpoint of symmetry of a tetragonal crystal.

Furthermore, the crystallographic faces of the {110} and {100} facesorthogonal to any one of the above crystal axes are polished, andprecise orientation is determined by a X-ray direction finder or thelike to correct the offset of the polished faces.

(3) Rough Cutting (Preparation of Wafer or Block With AppropriateThickness):

The single crystal ingot is cut with a cutting machine such as a wiresaw or an inner diamond saw in parallel with or substantiallyorthogonally to the polished faces {110} and {100} to obtain a plate(wafer) or a block having an appropriate thickness. After cutting,chemical etching with an etching solution may be performed as necessary.

(4) Polishing (Preparation of Wafer With Appropriate Thickness):

The wafer is ground or polished with a lapping machine, a polishingmachine, or a grinding machine to prepare the wafer with a predeterminedthickness. It should be noted that after grinding or polishing, chemicaletching with an etching solution may be performed as necessary.

(5) Fabrication of Single Crystal Device:

The wafer of the present invention has the (101) or (011) face as awafer face (widest face). As shown in FIGS. 5 and 6, the (101) face andthe (011) face are equivalent in view of symmetry of a tetragonalcrystal. A single crystal device material with a predetermined shape iscut out from the wafer using a precise cutting machine such as a dicingsaw or a cutting saw so that when the (101) face is the wafer face, thenormal direction 1 to the edge face T of the piezoelectric device 10A iswithin a solid-angle range ±25° with respect to the [-101] axis, therange including the [-101] axis, while when the {011} face is the waferface, the normal direction 1 to the edge face T of the piezoelectricdevice 10A is within a solid-angle range ±25° with respect to the [0-11]axis, the range including the [0-11] axis.

FIG. 5 shows, in a three-axis orthogonal coordinate system, the state ofcutting out the single crystal such that the (101) face is the waferface (widest face), and the normal direction 1 to the end face T of thepiezoelectric device 10A using the lateral vibration mode is the [-101]direction. FIG. 6 shows, in a three-axis orthogonal coordinate system,the state of cutting out the single crystal such that the (011) face isthe wafer face (widest face), and the normal direction 1 to the end faceT of the piezoelectric device 10B using the lateral vibration mode isthe [0-11] direction. The single crystal block may be directly cut outwith the dimensions of the piezoelectric device using a precisioncutting machine such as a dicing saw or a cutting saw.

(6) Formation of Electrode:

It is necessary to previously form electrodes required for applying anelectric field in the primary polarization processing or the auxiliarypolarization processing.

Before the primary polarization processing, a Cr—Au film (first Crlayer: about 50 nm in thickness, second Au layer: about 100 to 200 nm inthickness) is formed by sputtering on each of the top and bottom faces(the (101) and (-10-1) faces in FIG. 5 or the (011) and (0-1-1) faces inFIG. 6) of the prepared single crystal device material, a gold film isformed by plasma evaporation, or a silver film is formed by screenprinting, followed by baking to form the electrodes.

Before the auxiliary polarization processing, the electrodes are formedon two opposing faces perpendicular to the auxiliary polarizationdirection by the same method as described above.

When the primary polarization processing is performed after theauxiliary polarization processing or when the auxiliary polarizationprocessing is performed after the primary polarization processing, it isnecessary to completely remove the residual electrodes using anappropriate chemical etching solution or acid because the residualelectrodes used in the first polarization processing may destabilize thesubsequent polarization processing.

(7) Primary Polarization Processing:

In the single crystal cut out from the single crystal ingot aftergrowth, domains each composed of a group of electric dipoles in the samedirection have different electric dipole directions in the polarizationdirection 3 and the direction orthogonal thereto. Therefore, the singlecrystal does not exhibit piezoelectric properties and are in anunpolarized state.

Hence, polarization is required. For the piezoelectric device with thecomposition of the present invention, it is preferable to apply a DCelectric field of 350 to 1500 V/mm to the single crystal ingot, the cutout single crystal block, or the cut out single crystal device in thepolarization direction 3 in the temperature range of 20° C. to 200° C.In other words, when the polarization temperature is less than 20° C. orthe electric field is less than 350 V/mm, the polarization may beinsufficient, while when the temperature exceeds 200° C. or the electricfield exceeds 1500 V/mm, overpolarization may occur to deteriorate thepiezoelectric properties of the piezoelectric single crystal device.Furthermore, distortion in the crystal may increase due to an excessiveelectric field, leading to cracks in the piezoelectric single crystaldevice.

The polarization time is preferably adjusted according to thepolarization temperature and applied electric field selected in theabove-described preferred ranges. The maximum polarization time ispreferably 180 minutes.

Alternatively, it is preferable to decrease the temperature (electricfield cooling) to room temperature while applying a DC electric field of250 to 500 V/mm in the polarization direction 3 at a temperature higherthan the Curie temperature Tc (e.g., the Tc line in FIG. 4) of thesingle crystal device, preferably 190° C. to 200° C. Temperature risingto above the Curie temperature Tc temporarily eliminates electricdipoles, and temperature lowering to below the Curie temperature withthe electric field applied more uniformly aligns the directions of theelectric dipoles. In the case of a temperature below the Curietemperature, the electric dipoles partially remain, thereby causinginsufficient polarization. When the electric field is less than 250V/mm, polarization may be insufficient, while when the electric fieldexceeds 500 V/mm, overpolarization may occur. The cooling rate ispreferably determined so as not to cause cracks in the device duringcooling.

Note that the Curie temperature Tc is a transition temperature abovewhich a material exhibits neither piezoelectric properties norferroelectric properties since electric dipoles face in randomdirections and are not aligned. The Curie temperature is determined bythe composition and structure of a material (refer to the Tc line inFIG. 4).

(8) Auxiliary Polarization Processing:

While the above-described primary polarization processing is performedfor primary polarization of the piezoelectric single crystal device, theauxiliary polarization processing is performed for controlling thealignment state of a ferroelectric domain orthogonal to the polarizationdirection 3 by applying an electric field in the direction orthogonal tothe polarization direction 3, preferably the lateral vibration direction1, before or after the primary polarization processing.

Types of the electric field applied in the direction orthogonal to thepolarization direction 3 include attenuation electric fields andsteady-state electric fields such as DC electric fields, pulse electricfields, alternating current electric fields, and the like. Theappropriate conditions of the electric field, such as strength,application time, temperature, and the like depend on the properties ofeach piezoelectric single crystal device and the desiredelectromechanical coupling factor k₃₁ in the direction orthogonal to thepolarization direction. These conditions can be determined byexperiments or the like. In order to obtain the advantage of theauxiliary polarization, the auxiliary polarization processingtemperature is preferably 25° C. to a phase transition temperature (forexample, the Trt line shown in FIG. 4), and the applied electric fieldrange is preferably 350 to 1500 V/mm. It should be noted that thepolarization time is preferably adjusted depending on the polarizationtemperature and applied electric field selected in the above-describedpreferable ranges, in particular, preferably 10 minutes to 2 hours.

Also, examples of the pulse electric field include unipolar and dipolarpulses, such as alternating current triangle waves and like, as shown inFIG. 7, in addition to orthogonal waves.

EXAMPLE 1

The single crystal devices used in this example were the single crystaldevices 10A and 10B each composed of lead magnesium niobate (PMN) andlead titanate (PT) (PMN-PT) (composition formula: Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ (X=0.36)). The shapes and the like (Curie temperatureTc=186° C., device size: 13 mm length×4 mm width×0.36 mm thickness) ofthe piezoelectric single crystal device 10A having the [101] directionas the polarization direction and the piezoelectric single crystaldevice 10B having the [011] direction as the polarization direction areshown in FIGS. 8A and 8B, respectively.

The piezoelectric single crystal devices 10A and 10B were fabricated asfollows: A raw material was prepared to have the composition Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ (X=0.36), and then a single crystal ingot wasproduced by the aforementioned melt Bridgemann method. Next, the precisecrystallographic orientation of the single crystal ingot was determined,and the single crystal ingot was polished and then cut with a wire sawin parallel with the polished faces, i.e., the (101) face and the (011)face, to obtain a plate of 0.5 mm in thickness. The plate was polishedwith a polishing machine to obtain a wafer of 0.36 mm in thickness.Then, a device size of 13 mm length×4 mm width×0.36 mm thickness was cutout from the wafer using a dicing saw.

In cutting out the device shape with a dicing saw, in the piezoelectricsingle crystal device 10A having the (101) face as the polished face,the [101] axis of the tetragonal system was in the polarizationdirection 3, while in the piezoelectric single crystal device 10B havingthe (011). face as the polished face, the [011] axis of the tetragonalsystem was in the polarization direction 3. In each of the piezoelectricsingle crystal devices 10A and 10B, the orientation (strictly, thenormal direction 1 to the edge face T) of the edge face T was changed.Specifically, in the piezoelectric single crystal device 10A, as shownin FIG. 9A, the normal direction 1 to the edge face T (10 c) of thepiezoelectric single crystal device was changed from 0° ([-101]direction) to 90° ([010] direction) in increments of 5° in order toexamine the magnitude of the electromechanical coupling factor k₃₁. Onthe other hand, in the piezoelectric single crystal device 10B, as shownin FIG. 9B, the normal direction 1 to the edge face T (10c) of thepiezoelectric single crystal device was changed from 0° ([0-11]direction) to 90° ([100] direction) in increments of 5° in order toexamine the magnitude of the electromechanical coupling factor k₃₁.Then, a Cr—Au film (first Cr layer: about 50 nm in thickness, second Aulayer: about 100 to 200 nm in thickness) was formed by sputtering on theopposing top and bottom faces 10 a and 10 b of each of the fabricatedsingle crystal devices to form gold electrodes. The polarization wasperformed at 25° C. in air under a condition in which a DC electricfield of 700 V/mm was applied for 60 minutes in the directions ([101]direction and [011] direction) perpendicular to the drawings of FIGS. 9Aand 9B, respectively. The electromechanical coupling factors k₃₁ of thepiezoelectric single crystal devices 10A and 10B were calculatedaccording to the known arithmetic expression (refer to ElectronicMaterial Industrial Standard: EMAS-6008, 6100). The measured results areshown in Tables 1 and 2.

The reason for selecting the range of 0° to 90° with respect to the[-101] axial direction (FIG. 9A) or the [0-11] axial direction (FIG. 9B)in the plane substantially orthogonal to the polarization direction 3 isthat this angle range is necessary and sufficient to obtain informationrelating to all directions within the crystal face orthogonal to thepolarization direction in view of symmetry of a tetragonal crystal. Forthe purpose of reference, the electromechanical coupling factor k₃₁ of apiezoelectric device fabricated as a conventional example from asintered body ceramic (PZT) of lead zirconate titanate (Pb(Zr, Ti)O₃) isalso shown in Tables 1 and 2. Unlike the piezoelectric single crystaldescribed herein, PZT is a sintered body ceramic and does not haveanisotropy accompanying crystal orientation, and thus theelectromechanical coupling factor k₃₁ in the lateral vibration mode isthe same value in all crystal orientations regardless of the normaldirection 1 to the edge face T (10 c).

The results shown in Table 1 reveal that only within the angle range of0° to 25° (equivalent to the angle range −25° to +25° in view ofsymmetry of a crystal (tetragonal)), within the face orthogonal to thepolarization direction 3 including the [-101] axis, the piezoelectricsingle crystal device 10A exhibits an electromechanical coupling factork₃₁ of 50% or more and is suitable as a piezoelectric device employinglateral vibration.

Also, the results shown in Table 2 reveal that only within the anglerange of ±25° with respect to the [0-11] axis within the face orthogonalto the polarization direction 3 including the [0-11] axis, thepiezoelectric single crystal device 10B exhibits an electromechanicalcoupling factor k₃₁ of 50% or more and is suitable as a piezoelectricdevice employing lateral vibration.

Furthermore, as a result of detailed measurement of theelectromechanical coupling factor k₃₁ within this angle range inincrements other than 5°, it was confirmed that k₃₁ is consistently 50%or more.

Furthermore, in this example, with the [101] axis of the single crystaldevice as the polarization direction, confirmation was made ofpreferable orientation in which the (101) face as the widest face of thepiezoelectric single crystal device of 13 mm length×4 mm width×0.36 mmis orthogonal to the [101] direction. Alternatively, with the [011] axisof the single crystal device as the polarization direction, confirmationwas made of preferable orientation in which the (011) face as the widestface of the piezoelectric single crystal device of 13 mm length×4 mmwidth×0.36 mm is orthogonal to the [011] direction. As a result, k₃₁ wasas high as 54.3% at an angle of +15° with respect to the [-101] axis onthe (010) face orthogonal to the (101) face in which the normaldirection 1 to the edge face T shown in FIG. 5 is within the solid-anglerange of ±25° with respect to the [-101] axis. Also, k₃₁ was as high as55.2% at an angle of +15° with respect to the [0-11] axis on the (100)face orthogonal to the (011) face in which the normal direction 1 to theedge face T shown in FIG. 6 is within the solid-angle range of ±25° withrespect to the [0-11] axis.

A piezoelectric single crystal device was fabricated using lead indiummagnesium niobate (PIMN) and lead titanate (PT) (65PIMN-35PT) by thesame method as described above, and the electromechanical couplingfactor k₃₁ was examined under the same test conditions as describedabove. As a result, it was confirmed from Tables 1 and 2 that like with64PMN-36PT, a high electromechanical coupling factor k₃₁ can beobtained. In this case, the indium content was 20 mol %.

EXAMPLE 2

Next, as the piezoelectric single crystal device 10A with the [101]direction as the polarization direction and the piezoelectric singlecrystal device 10B with the [011] direction as the polarizationdirection, the single crystal devices Nos. 1 to 11 composed of Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ with different Ti molar fractions shown in Tables 3and 4 were fabricated by the same method as in EXAMPLE 1, andelectromechanical factors k₃₁ were calculated by the same method as inEXAMPLE 1. The results are shown in Table 3 and 4. Each of theelectromechanical coupling factors k₃₁ shown in Tables 3 and 4 is anaverage value of each single crystal device with a sample number n=5.The compositions of the piezoelectric single crystal devise 10A and 10Bwere the same as in EXAMPLE 1. The piezoelectric single crystal devices10A and 10B were fabricated by cutting out single crystal devicematerials of a size of 13 mm length×4 mm width×0.36 mm using a dicingsaw to realize the orientations of the piezoelectric single crystaldevices 10A and 10B in which as in EXAMPLE 1, the normal direction 1 tothe piezoelectric device edge face T (10 c) was 0° with respect to the[-101] axis and the [0-11] axis, respectively.

The results shown in Tables 3 and 4 indicate that in any one of samplesof the piezoelectric single crystal devices 10A and 10B according to theinvention in which the Ti molar faction satisfies 0.30<X<0.40, anelectromechanical coupling factor k₃₁ of as high as 50% or more isstably obtained.

EXAMPLE 3

Next, a preferable polarization processing method for fabricating apiezoelectric single crystal device preferable for utilizing the lateralvibration mode will be described below. Table 5 shows the results ofmeasurement of the electromechanical coupling factors k₃₁ of thepiezoelectric single crystal device 10A with the [101] direction as thepolarization direction and the piezoelectric single crystal device 10Bwith the [011] direction as the polarization direction, thesepiezoelectric single crystal devices being fabricated under variouspolarization processing conditions. The fabricating method, the devicedimensions, and the test conditions for the piezoelectric single crystaldevices were the same as in EXAMPLE 1. The piezoelectric single crystaldevices 10A and 10B had the same composition as in EXAMPLE 1 and werefabricated by cutting out single crystal device materials of a size of13 mm length×4 mm width×0.36 mm using a dicing saw to realize theorientations of the piezoelectric single crystal devices 10A and 10B inwhich as in EXAMPLE 1, the normal direction 1 to the piezoelectricdevice edge face T (10 c) was 15° with respect to the [-101] axis andthe [0-11] axis, respectively.

In Table 5, Nos. (1) to (7) are piezoelectric single crystal devicesfabricated under polarization processing conditions in which a DCelectric field of 350 to 1500 V/mm was applied for 30 to 180 minuteswithin the temperature range of 25° C. to 60° C. In this case, with leadmagnesium niobate (PMN) and lead titanate (PT) (Ti molar fraction X: 36mol %), electromechanical coupling factors k₃₁ in the direction (lateralvibration mode) orthogonal to the polarization direction of a crystalpreferable for utilizing the lateral vibration mode were 51.6% to 61.0%with the piezoelectric single crystal device 10A and 52.0% to 61.2% withthe piezoelectric single crystal ice 10B. The electromechanicalcomplying factor k₃₁ was 50% or more with both piezoelectric simplecrystal devices 10A and 10B.

With respect to lead indium magnesium niobate (PIMN) and lead titanate(PT) (PIMN-PT), piezoelectric single crystal devices were fabricated bythe same method as that for lead magnesium niobate (PMN) and leadtitanate (PT), and electromechanical coupling factors k₃₁ were measuredunder the same test conditions as those for lead magnesium niobate (PMN)and lead titanate (PT). Table 5 indicates that a piezoelectric singlecrystal device exhibiting an electromechanical coupling factor k₃₁ of ashigh as 50% or more is obtained by applying a DC current of 350 to 1500V/mm in the temperature range of 25° C. to 60° C. With any one of thepiezoelectric single crystal devices having a composition containinglead magnesium niobate (PMN), lead titanate (PT) (Ti molar faction X: 45mol %), and In (20 mol %) in an appropriate range, the same result aswith the composition containing lead magnesium niobate (PMN) and leadtitanate (PT) (Ti molar fraction X: 36 mol %) was obtained.

The present invention includes a temperature range and electric fieldrange of primary polarization processing conditions out of theabove-descried preferable ranges. However, when the polarizationprocessing temperature of a single crystal device was 25° C. and theapplied electric field was 320 V/mm lower than the lower limit of thepreferable range of the present invention, in the 64PMN-36PT device andthe 65PIMN-35PT of examples of the invention, the electromechanicalcoupling factors k₃₁ of the piezoelectric single crystal devices 10A and10B were less than 50% in some cases.

In addition, when the temperature of a single crystal device was 40° C.and the applied electric field was 1700 V/mm over the upper limit of thepreferable range of the present invention, in the 64PMN-36PT device andthe 65PIMN-35PT of examples of the invention, the electromechanicalcoupling factors k₃₁ of the piezoelectric single crystal devices 10A and10B were less than 50% in some cases, and cracks occurred in thepiezoelectric single crystal devices during the application of theelectric field or immediately after the application in some examples.

Furthermore, when the temperature was decreased to room temperature (25°C.) in silicone oil over 120 minutes with the DC electric field of 400V/mm applied to a single crystal device in the silicone oil at 210° C.higher than the Curie temperature Tc shown in FIG. 4, with leadmagnesium niobate (PMN) and lead titanate (PT) (Ti molar fraction X: 36mol %), the electromechanical coupling factor k₃₁ in the direction(lateral vibration mode) orthogonal to the polarization direction was58.6% with the piezoelectric single crystal device 10A and 58.4% withthe piezoelectric single crystal ice 10B, as shown in No. (8) in Table5. With respect to a piezoelectric single crystal device (PIMN-PT)having a composition containing lead magnesium niobate (PMN), leadtitanate (PT) (Ti molar fraction X: 45 mol %), and In (30 mol %) in anappropriate range, electromechanical coupling factors k₃₁ were measuredunder the same test conditions as those for lead magnesium niobate (PMN)and lead titanate (PT). As a result, as shown in No. (8) in Table 5, k₃₁was 57.3% with the piezoelectric single crystal device 10A and 58.1%with the piezoelectric single crystal device 10B and improved like withlead magnesium niobate (PNM) and lead titanate (PT).

These results indicate that the method of cooling with the electricfield applied (electric field cooling) is effective.

In the 64PMN-36PT device and the 65PIMN-35PT device of examples of theinvention, with the applied electric field less than 250 V/mm, theelectromechanical coupling factors k₃₁ of the piezoelectric singlecrystal devices 10A and 10B were less than 50% in some cases. This ispossibly due to the fact that the electric field less than 200 V/mmcauses insufficient polarization. On the other hand, in the 64PMN-36PTdevice and the 65PIMN-35PT device of examples of the invention, with theapplied electric field over 500 V/mm, the electromechanical couplingfactors k₃₁ of the piezoelectric single crystal devices 10A and 10B wereless than 50% in some cases, and cracks occurred in the piezoelectricsingle crystal devices during the application of the electric field of600 V/mm or immediately after the application in some cases.

As described above, in the 64PMN-36PT device and the 65PIMN-35PT deviceof examples of the invention, a satisfactory electromechanical couplingfactor k₃₁ is stably obtained with both the piezoelectric single crystaldevices 10A and 10B under the preferable polarization conditions of theinvention.

EXAMPLE 4

Next, a preferable auxiliary polarization processing method forfabricating a piezoelectric single crystal device preferable forutilizing the lateral vibration mode will be described below. Table 6shows the results of measurement of the electromechanical couplingfactors k₃₁ of piezoelectric single crystal devices fabricated undervarious auxiliary polarization processing conditions. The fabricatingmethod, the device dimensions, and the test conditions for thepiezoelectric single crystal devices were the same as in EXAMPLE 1. Thepiezoelectric single crystal devices 10A and 10B had the samecomposition as in EXAMPLE 1 and were fabricated by cutting out singlecrystal device materials of a size of 13 mm length×4 mm width×0.36 mmusing a dicing saw to realize the orientations of the piezoelectricsingle crystal devices 10A and 10B in which as in EXAMPLE 1, the normaldirection 1 to the piezoelectric device edge face T (10 c) was 15° withrespect to the [-101] axis and the [0-11] axis, respectively.

A Cr—Au film (first Cr layer: about 50 nm in thickness, second Au layer:about 100 to 200 nm in thickness) was formed by sputtering on each ofboth edge faces 10 c (T) of a crystal suitable for utilizing the lateralvibration mode, which was fabricated by the same method as in EXAMPLE 1to form electrodes. Then, auxiliary polarization processing wasperformed by applying a DC electric field of 320 to 1700 V/mm for a timeof 10 minutes to 150 minutes at an auxiliary polarization processingtemperature of 25° C. to 40° C. Then, the electrodes were completelyremoved by dissolution with an etching solution or an acid, and a Cr—Aufilm (first Cr layer: about 50 nm in thickness, second Au layer: about100 to 200 nm in thickness) was formed by sputtering on each of theopposing top and bottom faces 10 a and 10 b of a single crystal devicematerial 10 to form electrodes, and main polarization processing wasperformed by applying a DC electric field of 700 V/mm for 60 minutes at25° C. in air. The measured electromechanical coupling factors k₃₁ areshown in Table 6. In Table 6, Nos. (1) to (5) were piezoelectric singlecrystal devices fabricated under auxiliary polarization processingconditions in which a DC electric field of 350 to 1500 V/mm was appliedfor 10 to 120 minutes in the temperature range of 25° C. to 40° C.

In this case, with lead magnesium niobate (PMN) and lead titanate (PT)(Ti molar fraction X: 36 mol %), the electromechanical coupling factork₃₁ was 60% or more with both piezoelectric single crystal devices 10Aand 10B, as compared with No. (9) in Table 6 without auxiliarypolarization processing in which k₃₁ was 59.8% with the piezoelectricsingle crystal device 10A and 59.1% with the piezoelectric singlecrystal device 10B. Therefore, the electromechanical coupling factor k₃₁was further increased by the auxiliary polarization processing. Also, inNo. (6) in which auxiliary polarization processing was performed underthe same conditions as described above in (2) after primary polarizationprocessing, the electromechanical coupling factor k₃₁ was 62.4% with thepiezoelectric single crystal device 10A and 62.8% with the piezoelectricsingle crystal device 10B, and high electromechanical coupling factorsk₃₁ were obtained.

Furthermore, in Nos. (7) and (8) in which a bipolar triangular pulseelectric field shown in FIG. 7 was applied for 10 minutes before orafter the primary polarization processing, high electromechanicalcoupling factors k₃₁ were obtained.

With respect to a composition containing lead magnesium niobate (PMN),lead titanate (PT) (Ti molar fraction X: 45 mol %), and In (30 mol %) inan appropriate range, piezoelectric single crystal devices (PIMN-PT)were fabricated by the same method as that for lead magnesium niobate(PMN) and lead titanate (PT), and electromechanical coupling factors k₃₁were measured under the same test conditions as those for lead magnesiumniobate (PMN) and lead titanate (PT). Nos. (1) to (8) in Table 6indicate that like in the case of lead magnesium niobate (PMN) and leadtitanate (PT), with a crystal suitable for utilizing the lateralvibration mode, the electromechanical coupling factor k₃₁ was improvedby auxiliary polarization processing before or after primarypolarization processing under the auxiliary polarization processingconditions in which a DC electric field of 350 to 1500 V/mm or a bipolartriangular pulse electric field was applied at the temperature range of25° C. to 40° C.

On the other hand, when the auxiliary polarization processingtemperature of a piezoelectric single crystal device was 25° C. and theapplied electric field was 320 V/mm lower than the lower limit of thepreferable range of the invention, in the 64PMN-36PT device and the65PIMN-35PT device of examples of the invention, the electromechanicalcoupling factors k₃₁ of the piezoelectric single crystal devices 10A and10B were less than 50% in some cases.

Furthermore, when the temperature of a piezoelectric single crystaldevice fabricated by the same method as in EXAMPLE 1 was 40° C. and theapplied electric field was 1700 V/mm over the upper limit of thepreferable range of the invention, in the 64PMN-36PT device and the65PIMN-35PT device of examples of the invention, the electromechanicalcoupling factors k₃₁ of the piezoelectric single crystal devices 10A and10B were less than 50% in some cases, and cracks occurred in thepiezoelectric single crystal devices in some cases.

EXAMPLE 5

Next, according to the present invention, various types of thepiezoelectric single crystal devices 10A and 10B composed of leadmagnesium niobate (PNM) and lead titanate (PT) (PMN-PT) (compositionformula: Pb[(Mg, Nb)_(1-X)Ti_(X)]O₃) in which the Ti molar fraction Xsatisfies 0.30<X<0.40 (over 30 mol % and less than 40 mol %) werefabricated, and changes in dielectric constant with temperature weremeasured to determine the Curie temperatures Tc and the phase transitiontemperatures T_(rt). The piezoelectric single crystal devices 10A and10B were fabricated by cutting out single crystal device materials of asize of 13 mm length×4 mm width×0.36 mm using a dicing saw to realizethe orientations of the piezoelectric single crystal devices 10A and 10Bin which as in EXAMPLE 1, the normal direction 1 to the piezoelectricdevice edge face T (10 c) was 0° with respect to the [-101] axis and the[0-11] axis, respectively.

The results are shown in FIG. 10. For the purpose of reference, variouspiezoelectric single crystal devices having a Ti molar fraction X of 0.3or less (30 mol % or less) were fabricated by the same method as theabove, and changes in dielectric constant with temperature were measuredto determine the Curie temperatures Tc and the phase transitiontemperatures T_(rt). The results are shown in FIG. 11.

As shown in FIG. 10, in any one of the piezoelectric single crystaldevices 10A and 10B of the invention in which the Ti molar fraction Xsatisfies 0.30<X<0.40 (over 30 mol % and less than 40 mol %), the Curietemperature is Tc is as high as 160° C. or more, and the phasetransition temperature T_(rt) is room temperature or less because atetragonal structure is superior to a pseudocubic structure. This meansthat within the temperature range of room temperature to a hightemperature (e.g., 150° C.), the same crystal structure (tetragonal) canbe maintained at room temperature and a high temperature without phasetransition, and thus the piezoelectric properties of the piezoelectricsingle crystal devices 10A and 10B are little deteriorated at a hightemperature.

On the other hand, as shown in FIG. 11, in the piezoelectric singlecrystal devices having a Ti molar fraction X of 0.3 or less (30 mo % orless), the Curie temperature Tc is as low as 130° C. to 155° C., and thephase transition temperature T_(rt) is as low as 90° C. or less. Thismeans that within the temperature range of room temperature to a hightemperature (e.g., 150° C.), a rhombohedral structure is superior atroom temperature, and phase transition to a superior tetragonalstructure occurs at a high temperature. Namely, the same crystalstructure (tetragonal) cannot be maintained at room temperature and ahigh temperature, and thus the piezoelectric properties of thepiezoelectric single crystal device 10A and 10B are deteriorated at ahigh temperature.

FIG. 12A is a plot diagram showing changes in the value of theelectromechanical coupling factor k₃₁ in repeated heat cycle testsbetween room temperature and high temperatures (e.g., 100° C., 120° C.,140° C.) using the piezoelectric single crystal device 10A of theinvention in which the Ti molar fraction X satisfies 0.30<X<0.40 (over30 mol % and less than 40 mol %). FIG. 12B is a plot diagram showingchanges in the value of the electromechanical coupling factor k₃₁ in thelateral vibration mode in repeated heat cycle tests between roomtemperature and high temperatures (e.g., 100° C., 120° C., 140° C.)using the piezoelectric single crystal device 10B of the invention inwhich the Ti molar fraction X satisfies 0.30<X<0.40 (over 30 mol % andless than 40 mol %). A heat cycle test was performed under theconditions in which the temperature was increased from room temperatureto each of 100° C. (retention time: 60 minutes), 120° C. (retentiontime: 30 minutes), and 140° C. (retention time: 15 minutes) andmaintained at each temperature and then decreased to room temperature,followed by measurement. Then, a heat treatment was further performedunder the conditions. For the purpose of reference, FIG. 13 is a plotdiagram showing changes in the value of the electromechanical couplingfactor k₃₁ in the lateral vibration mode in repeated heat cycle testsusing a piezoelectric single crystal device in which the Ti molarfraction X 0.30 or less (30 mol % or less).

These figures indicate that in any one of the piezoelectric singlecrystal devices 10A and 10B of the invention in which the Ti molarfraction X satisfies 0.30<X<0.40 (over 30 mol % and less than 40 mol %),the value of the electromechanical coupling factor k₃₁ in the lateralvibration mode little changes in the repeated heat cycle tests.

On the other hand, FIG. 13 shows that in the piezoelectric singlecrystal device in which the Ti molar fraction X is 0.30 or less (30 mol% or less), the value of the electromechanical coupling factor k₃₁ inthe lateral vibration mode significantly decreases in the repeated heatcycle tests.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to fabricate apiezoelectric single crystal device used in applications positivelyutilizing an electromechanical coupling factor k₃₁ in a direction(lateral vibration mode) orthogonal to the polarization direction, forexample, magnetic head precise positioning actuators, piezoelectric gyrodevices, image stabilizers of digital still cameras, cardiac pacemakersensors, and the like. In particular, a piezoelectric single crystaldevice of the present invention is capable of stably maintaining theelectromechanical coupling factor k₃₁ in the lateral vibration mode at ahigh value without a decrease even in a high-temperature (specifically,150° C.) operating environment.

TABLE 1 Electromechanical coupling factor k₃₁ of piezoelectric singlecrystal device 10A (%) 65PIMN-35PT Crystal Angle 64PMN-36PT (Ti: 35 mol%) orientation (°) (Ti: 36 mol %) (In: 20 mol %) Pb(Zr,Ti)O₃ [−101] 062.0 61.5 30 5 62.1 62.3 10 61.8 61.3 15 59.8 58.2 20 54.3 55.4 25 51.352.0 30 41.2 39.8 35 30.0 30.1 40 28.3 27.6 45 27.3 27.0 50 27.1 26.7 5525.1 24.3 60 26.8 24.2 65 24.8 23.0 70 24.9 24.9 75 25.0 25.1 80 23.723.3 85 23.5 23.6 [010] 90 23.8 23.7

TABLE 2 Electromechanical coupling factor k₃₁ of piezoelectric singlecrystal device 10B (%) 65PIMN-35PT Crystal Angle 64PMN-36PT (Ti: 35 mol%) orientation (°) (Ti: 36 mol %) (In: 20 mol %) Pb(Zr,Ti)O₃ [0-11] 062.1 61.8 30 5 62.0 61.9 10 61.3 60.3 15 59.1 60.2 20 54.2 53.6 25 51.451.2 30 42.6 41.6 35 31.0 31.3 40 29.6 29.5 45 26.1 25.8 50 26.3 26.0 5525.2 25.0 60 26.0 24.6 65 24.2 24.1 70 24.8 24.2 75 23.7 23.0 80 23.623.2 85 23.5 23.1 [100] 90 23.8 23.5

TABLE 3 Piezoelectric single crystal device 10A Evaluation result PMNElectromechanical Sample mol PT Ti coupling factor k₃₁ No. % mol % mol %(%) Remarks 1 72.0 28.0 28.0 47.3 Comparative Example 2 69.9 30.1 30.153.0 Example of this invention 3 67.9 32.1 32.1 57.0 Example of thisinvention 4 66.7 33.3 33.3 59.8 Example of this invention 5 64.3 35.635.6 60.3 Example of this invention 6 63.7 36.3 36.3 61.8 Example ofthis invention 7 62.6 37.4 37.4 62.0 Example of this invention 8 61.538.5 38.5 57.0 Example of this invention 9 60.1 39.9 39.9 53.0 Exampleof this invention 10 59.5 40.5 40.5 46.8 Comparative Example 11 56.044.0 44.0 40.2 Comparative Example

TABLE 4 Piezoelectric single crystal device 10B Evaluation result PMNElectromechanical Sample mol PT Ti coupling factor k₃₁ No. % mol % mol %(%) Remarks 1 71.9 28.1 28.1 47.6 Comparative Example 2 69.7 30.3 30.353.2 Example of this invention 3 67.4 32.6 32.6 57.3 Example of thisinvention 4 66.5 33.5 33.5 59.7 Example of this invention 5 64.3 35.735.7 60.5 Example of this invention 6 63.7 36.3 36.3 61.8 Example ofthis invention 7 62.5 37.5 37.5 62.1 Example of this invention 8 61.438.6 38.6 57.3 Example of this invention 9 59.9 39.9 39.9 53.2 Exampleof this invention 10 59.7 40.3 40.3 46.3 Comparative Example 11 56.044.0 44.0 39.8 Comparative Example

TABLE 5 Electromechanical coupling factor k₃₁ (%) 64PMN- 65PIMN-35PTPolarization condition 36PT (Ti: (Ti: 35 mol %) Temperature Electricfield Time 36 mol %) (In: 20 mo %) Remarks ° C. V/mm min 10A 10B 10A 10BExample of this invention (1) 25 350 180 51.6 52.0 51.5 50.8 Example ofthis invention (2) 60 400 180 56.8 56.5 56.4 56.9 Example of thisinvention (3) 25 700 100 60.6 61.2 60.4 60.8 Example of this invention(4) 25 700 60 59.8 59.1 59.2 59.3 Example of this invention (5) 40 90070 61.0 60.3 60.4 60.4 Example of this invention (6) 30 1200 60 60.960.8 61.0 60.4 Example of this invention (7) 40 1500 30 59.6 57.8 58.358.4 Example of this invention (8) 210→25 400 120 58.6 58.4 57.3 58.1Example of this invention electric field cooling 10A: Piezoelectricsingle crystal device 10A 10B: Piezoelectric single crystal device 10B

TABLE 6 Electromechanical coupling Auxiliary polarization conditionfactor k₃₁ (%) Timing 64PMN- 65PIMN-35PT Type of Electric of 36PT (Ti:(Ti: 35 mol %) Temp. electric field Time auxiliary 36 mol %) (In: 20mo%) Remarks ° C. field V/mm min polarization 10A 10B 10A 10B Example ofthis invention (1) 40 DC 350 120 Pre- 60.4 60.3 60.2 60.4 Example ofthis invention processing (2) 25 DC 700 100 Pre- 62.5 63.0 62.7 62.1Example of this invention processing (3) 40 DC 900 70 Pre- 62.4 62.562.3 62.5 Example of this invention processing (4) 30 DC 1200 60 Pre-61.8 61.8 61.3 61.5 Example of this invention processing (5) 40 DC 150010 Pre- 61.4 60.8 60.4 61.3 Example of this invention processing (6) 25DC 700 100 Post- 62.4 62.8 62.3 62.4 Example of this inventionprocessing (7) 25 Triangular Peak value Pre- 61.2 60.8 60.4 61.0 Exampleof this invention wave 500 V/mm, processing (8) 25 pulse interval Post-61.4 60.9 60.5 60.6 Example of this invention 800 msec, 10 minprocessing (9) Unprocessed 59.8 59.1 59.2 59.3 Example of this invention10A: Piezoelectric single crystal device 10A 10B: Piezoelectric singlecrystal device 10B

1. A piezoelectric single crystal device; wherein with a polarizationdirection along a [101] axis of a tetragonal system having a [001] axisas a C axis (with the largest lattice constant), a normal direction toan edge face thereof is within the solid-angle range of ±25° withrespect to a [-101] axis which is substantially orthogonal to thepolarization direction, the range including the [-101] axis; and theelectromechanical coupling factor k₃₁ in a direction substantiallyorthogonal to the polarization direction, i.e., in a lateral vibrationmode, is 50% or more.
 2. A piezoelectric single crystal device; whereinwith a polarization direction along a [011] axis of a tetragonal systemhaving a [001] axis as a C axis (with the largest lattice constant), anormal direction to an edge face thereof is within the solid-angle rangeof ±25° with respect to a [0-11 ] axis which is substantially orthogonalto the polarization direction, the range including the [0-11] axis; andthe electromechanical coupling factor k₃₁ in a direction substantiallyorthogonal to the polarization direction, i.e., in a lateral vibrationmode, is 50% or more.
 3. The piezoelectric single crystal deviceaccording to claim 1, comprising a single crystal device material whichis a solid solution of Pb[(Mg, Nb)_(1-X)Ti_(X)]O₃ (wherein X is themolar fraction of Ti relative to the total molar fraction of 1 of Mg,Nb, and Ti), X satisfying the relation 0.30<X<0.40 and the materialhaving a complex perovskite structure.
 4. The piezoelectric singlecrystal device according to claim 1, wherein the solid solution furthercontains 0.05 mol % to 30 mol % of In.
 5. A method for fabricating thepiezoelectric single crystal according to claim 1, the methodcomprising: primary polarization processing for polarizing a singlecrystal ingot, a cut out single crystal block, or a cut out singlecrystal device by applying an electric field under predeterminedconditions in a polarization direction along a [101] or [011] directionbefore or after a single crystal device material of a predeterminedshape is cut out from the single crystal ingot in a predetermineddirection.
 6. The method for fabricating the piezoelectric singlecrystal device according to claim 5, the method comprising: primarypolarization processing for polarizing the single crystal ingot or thesingle crystal block by applying an electric field in the [101] or [011]direction under predetermined conditions; and processing for cutting outa single crystal device of a predetermined shape in a predetermineddirection from the single crystal ingot or the single crystal block. 7.The method for fabricating the piezoelectric single crystal deviceaccording to claim 5, wherein the primary polarization processingincludes applying a DC electric field of 350 to 1500 V/mm in thetemperature range of 20° C. to 200° C. in the [101] or [011] directionof the single crystal ingot or the single crystal block or cooling thesingle crystal ingot or the single crystal block to room temperaturewhile applying a DC electric field of 250 to 500 V/mm at a temperaturehigher than the Curie temperature (Tc) thereof.
 8. The method forfabricating the piezoelectric single crystal device according to claim5, the method comprising: processing for cutting out a single crystaldevice of a predetermined shape in a predetermined direction from thesingle crystal ingot; and primary polarization processing for polarizingthe single crystal device by applying an electric field in the [101] or[011] direction under predetermined conditions.
 9. The method forfabricating the piezoelectric single crystal device according to claim5, wherein the primary polarization processing includes applying a DCelectric field of 350 to 1500 V/mm in the temperature range of 20° C. to200° C. in the [101] or [011] direction of the single crystal device orcooling the single crystal device to room temperature while applying aDC electric field of 250 to 500 V/mm at a temperature higher than theCurie temperature (Tc) thereof.
 10. The method for fabricating thepiezoelectric single crystal device according to claim 5, furthercomprising auxiliary polarization processing for polarization byapplying an electric field in a direction orthogonal to the polarizationdirection before or after the primary polarization processing.
 11. Thepiezoelectric single crystal device according to claim 2, comprising asingle crystal device material which is a solid solution of Pb[(Mg,Nb)_(1-X)Ti_(X)]O₃ (wherein X is the molar fraction of Ti relative tothe total molar fraction of 1 of Mg, Nb, and Ti), X satisfying therelation 0.30<X<0.40 and the material having a complex perovskitestructure.
 12. The piezoelectric single crystal device according toclaim 2, wherein the solid solution further contains 0.05 mol % to 30mol % of In.
 13. The piezoelectric single crystal device according toclaim 3, wherein the solid solution further contains 0.05 mol % to 30mol % of In.
 14. The piezoelectric single crystal device according toclaim 11, wherein the solid solution further contains 0.05 mol % to 30mol % of In.
 15. The method for fabricating the piezoelectric singlecrystal according to claim 2, the method comprising: primarypolarization processing for polarizing a single crystal ingot, a cut outsingle crystal block, a cut out single crystal block, or a cut outsingle crystal device by applying an electric field under predeterminedconditions in a polarization direction along a [101] or [011] directionbefore or after a single crystal device material of a predeterminedshape is cut out from the single crystal ingot in a predetermineddirection.
 16. The method for fabricating the piezoelectric singlecrystal according to claim 3, the method comprising: primarypolarization processing for polarizing a single crystal ingot, a cut outsingle crystal block, a cut out single crystal block, or a cut outsingle crystal device by applying an electric field under predeterminedconditions in a polarization direction along a [101] or [011] directionbefore or after a single crystal device material of a predeterminedshape is cut out from the single crystal ingot in a predetermineddirection.
 17. The method for fabricating the piezoelectric singlecrystal device according to claim 15, the method comprising: primarypolarization processing for polarizing the single crystal ingot or thesingle crystal block by applying an electric field in the [101] or [011]direction under predetermined conditions; and processing for cutting outa single crystal device of a predetermined shape in a predetermineddirection from the single crystal ingot or the single crystal block. 18.The method for fabricating the piezoelectric single crystal deviceaccording to claim 16, the method comprising: primary polarizationprocessing for polarizing the single crystal ingot or the single crystalblock by applying an electric field in the [101] or [011] directionunder predetermined conditions; and processing for cutting out a singlecrystal device of a predetermined shape in a predetermined directionfrom the single crystal ingot or the single crystal block.
 19. Themethod for fabricating the piezoelectric single crystal device accordingto claim 6, wherein the primary polarization processing includesapplying a DC electric field of 350 to 1500 V/mm in the temperaturerange of 20° C. to 200° C. in the [101] or [011] direction of the singlecrystal ingot or the single crystal block or cooling the single crystalingot or the single crystal block to room temperature while applying aDC electric field of 250 to 500 V/mm at a temperature higher than theCurie temperature (Tc) thereof.
 20. The method for fabricating thepiezoelectric single crystal device according to claim 17, wherein theprimary polarization processing includes applying a DC electric field of350 to 1500 V/mm in the temperature range of 20° C. to 200° C. in the[101] or [011] direction of the single crystal ingot or the singlecrystal block or cooling the single crystal ingot or the single crystalblock to room temperature while applying a DC electric field of 250 to500 V/mm at a temperature higher than the Curie temperature (Tc)thereof.
 21. The method for fabricating the piezoelectric single crystaldevice according to claim 18, wherein the primary polarizationprocessing includes applying a DC electric field of 350 to 1500 V/mm inthe temperature range of 20° C. to 200° C. in the [101] or [011]direction of the single crystal ingot or the single crystal block orcooling the single crystal ingot or the single crystal block to roomtemperature while applying a DC electric field of 250 to 500 V/mm at atemperature higher than the Curie temperature (Tc) thereof.
 22. Themethod for fabricating the piezoelectric single crystal device accordingto claim 15, the method comprising: processing for cutting out a singlecrystal device of a predetermined shape in a predetermined directionfrom the single crystal ingot; and primary polarization processing forpolarizing the single crystal device by applying an electric field inthe [101] or [011] direction under predetermined conditions.
 23. Themethod for fabricating the piezoelectric single crystal device accordingto claim 16, the method comprising: processing for cutting out a singlecrystal device of a predetermined shape in a predetermined directionfrom the single crystal ingot; and primary polarization processing forpolarizing the single crystal device by applying an electric field inthe [101] or [011] direction under predetermined conditions.
 24. Themethod for fabricating the piezoelectric single crystal device accordingto claim 8, wherein the primary polarization processing includesapplying a DC electric field of 350 to 1500 V/mm in the temperaturerange of 20° C. to 200° C. in the [101] or [011] direction of the singlecrystal device or cooling the single crystal device to room temperaturewhile applying a DC electric field of 250 to 500 V/mm at a temperaturehigher than the Curie temperature (Tc) thereof.
 25. The method forfabricating the piezoelectric single crystal device according to claim22, wherein the primary polarization processing includes applying a DCelectric field of 350 to 1500 V/mm in the temperature range of 20° C. to200° C. in the [101] or [011] direction of the single crystal device orcooling the single crystal device to room temperature while applying aDC electric field of 250 to 500 V/mm at a temperature higher than theCurie temperature (Tc) thereof.
 26. The method for fabricating thepiezoelectric single crystal device according to claim 23, wherein theprimary polarization processing includes applying a DC electric field of350 to 1500 V/mm in the temperature range of 20° C. to 200° C. in the[101] or [011] direction of the single crystal device or cooling thesingle crystal device to room temperature while applying a DC electricfield of 250 to 500 V/mm at a temperature higher than the Curietemperature (Tc) thereof.
 27. The method for fabricating thepiezoelectric single crystal device according to claim 6, furthercomprising auxiliary polarization processing for polarization byapplying an electric field in a direction orthogonal to the polarizationdirection before or after the primary polarization processing.
 28. Themethod for fabricating the piezoelectric single crystal device accordingto claim 7, further comprising auxiliary polarization processing forpolarization by applying an electric field in a direction orthogonal tothe polarization direction before or after the primary polarizationprocessing.
 29. The method for fabricating the piezoelectric singlecrystal device according to claim 8, further comprising auxiliarypolarization processing for polarization by applying an electric fieldin a direction orthogonal to the polarization direction before or afterthe primary polarization processing.
 30. The method for fabricating thepiezoelectric single crystal device according to claim 9, furthercomprising auxiliary polarization processing for polarization byapplying an electric field in a direction orthogonal to the polarizationdirection before or after the primary polarization processing.